#region Apache License
//
// Licensed to the Apache Software Foundation (ASF) under one or more 
// contributor license agreements. See the NOTICE file distributed with
// this work for additional information regarding copyright ownership. 
// The ASF licenses this file to you under the Apache License, Version 2.0
// (the "License"); you may not use this file except in compliance with 
// the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
#endregion

using System;
using System.Collections;

namespace log4net.Util
{
  /// <summary>
  /// An always empty <see cref="IEnumerator"/>.
  /// </summary>
  /// <remarks>
  /// <para>
  /// A singleton implementation of the <see cref="IEnumerator"/> over a collection
  /// that is empty and not modifiable.
  /// </para>
  /// </remarks>
  /// <author>Nicko Cadell</author>
  /// <author>Gert Driesen</author>
  public sealed class NullEnumerator : IEnumerator
  {
    #region Private Instance Constructors

    /// <summary>
    /// Initializes a new instance of the <see cref="NullEnumerator" /> class. 
    /// </summary>
    /// <remarks>
    /// <para>
    /// Uses a private access modifier to enforce the singleton pattern.
    /// </para>
    /// </remarks>
    private NullEnumerator()
    {
    }

    #endregion Private Instance Constructors

    #region Public Static Properties

    /// <summary>
    /// Get the singleton instance of the <see cref="NullEnumerator" />.
    /// </summary>
    /// <returns>The singleton instance of the <see cref="NullEnumerator" />.</returns>
    /// <remarks>
    /// <para>
    /// Gets the singleton instance of the <see cref="NullEnumerator" />.
    /// </para>
    /// </remarks>
    public static NullEnumerator Instance
    {
      get { return s_instance; }
    }

    #endregion Public Static Properties

    #region Implementation of IEnumerator

    /// <summary>
    /// Gets the current object from the enumerator.
    /// </summary>
    /// <remarks>
    /// Throws an <see cref="InvalidOperationException" /> because the 
    /// <see cref="NullDictionaryEnumerator" /> never has a current value.
    /// </remarks>
    /// <remarks>
    /// <para>
    /// As the enumerator is over an empty collection its <see cref="Current"/>
    /// value cannot be moved over a valid position, therefore <see cref="Current"/>
    /// will throw an <see cref="InvalidOperationException"/>.
    /// </para>
    /// </remarks>
    /// <exception cref="InvalidOperationException">The collection is empty and <see cref="Current"/> 
    /// cannot be positioned over a valid location.</exception>
    public object Current
    {
      get { throw new InvalidOperationException(); }
    }

    /// <summary>
    /// Test if the enumerator can advance, if so advance
    /// </summary>
    /// <returns><c>false</c> as the <see cref="NullEnumerator" /> cannot advance.</returns>
    /// <remarks>
    /// <para>
    /// As the enumerator is over an empty collection its <see cref="Current"/>
    /// value cannot be moved over a valid position, therefore <see cref="MoveNext"/>
    /// will always return <c>false</c>.
    /// </para>
    /// </remarks>
    public bool MoveNext()
    {
      return false;
    }

    /// <summary>
    /// Resets the enumerator back to the start.
    /// </summary>
    /// <remarks>
    /// <para>
    /// As the enumerator is over an empty collection <see cref="Reset"/> does nothing.
    /// </para>
    /// </remarks>
    public void Reset()
    {
    }

    #endregion Implementation of IEnumerator

    #region Private Static Fields

    /// <summary>
    /// The singleton instance of the <see cref="NullEnumerator" />.
    /// </summary>
    private static readonly NullEnumerator s_instance = new NullEnumerator();

    #endregion Private Static Fields
  }
}
